Crystalline materials exhibit an ordered and repeated structure throughout the extent of the crystal. If a set of reference axes is defined at any point within the crystal, the atoms within the structure may be considered to lie in planes which intercept the reference axes at various distances. The number and types of planes which a crystal possesses, the spacing between the planes and the angles which these planes make with the reference axes are important characteristics of the particular crystal under study.
Crystal orientation is generally determined by X-ray diffraction techniques. Laue in 1913 postulated that the atoms in a crystal structure might serve as a three-dimensional diffraction grating for X-rays because experimentation had correctly indicated that X-ray wavelengths are on the order of 10.sup.-8 cm which is about the same as the interatomic spacing in the solid.
In practice, the surface of a crystal sample is irradiated by a collimated beam of X-radiation, and a flat, X-radiation sensitive film is exposed to the diffracted radiation to develop a diffraction pattern or "Laue photograph". The diffracted beams which form the diffraction pattern satisfy Bragg's Law, n.lambda.=2d sin .theta., where n is the order of refraction, .lambda. is the wavelength of the radiation in Angstrom units, d is the crystalline interplanar spacing in Angstrom units, and .theta. is the angle between the radiation and the normal of the crystalline planes. The resultant photograph is used to provide the information of crystal orientation.
In practice, however, analysis of the Laue photograph cannot be made directly from the diffraction pattern developed from the exposed film because the pattern is distorted as a result of gnomonic projection of diffraction spots on a unit sphere from a point (crystal) to a flat film along unequal paths. In order to compensate for this distortion, the angles obtained from the diffraction pattern spots on the photograph must be transformed onto a Wulff net by using a Greninger chart for standard stereographic projection examination.
The orientation of each spot is determined by trial and error by first assigning an orientation to one of the bright spots and trying to match the angles between the neighboring spots from the tabulated interplanar angles for the specific crystal structure. Bearing in mind that a typical Laue photograph may comprise 50 or more spots, the work is tedious and time consuming. The method becomes extremely difficult for indexing high index orientations as well as any orientation for structures of low symmetry. An alternative method is to reorient the crystal to the orientation of one of the bright spots from the first photograph and take another Laue photograph. The above procedure can then be repeated.
In view of all these disadvantages of the conventional Laue photography, a primary object of the present invention is to provide a new and improved Laue camera for direct crystal orientation determination with undistorted Laue photographs thereby eliminating the need for the Greninger chart, the stereographic projection and the work of matching interplanar angles.
Another object is to provide a new and improved Laue camera that is easier and faster to operate for a person with no previous experience in crystal orientation determination.